Any tunnel boat that doesn't have a full tunnel (meaning running the length of the entire hull) has a 'pocket' tunnel.

Here is a good description of the different types of hulls that run well in shallow water. It is from a study plans page for the designer of the boat I built.

Pocket drive, catamaran tunnel and some theory:
​

There are different ways to design a flats boat with minimal draft. Some catamarans claim to have less draft than monohulls but that is simply not possible.
Let's demonstrate by comparing the geometry of two simple hulls.
The sketch below shows the waterplane area of a monohull compared to a typical catamaran hull, each simple rectangles but the comparison works just as well for more sophisticated shapes.
​

You can see that the catamaran hull as less foot print than the monohull: at equal draft, the cat displaces only half the water of the mono hull
​

​

and therefore, at equal boat weight will have two times more draft. Even if the tunnel is narrower, the cat hull will always need more draft but there is more: a cat structure is more complicated and the hull area is larger, therefore heavier.
A cat type tunnel will always have more draft because it has less waterplane area and a heavier structure.
​

That is for static draft but what happens when running?
There again the monohull is superior.
The water between the hulls of the cat is turbulent. All kind of steps and other contraptions installed between the hulls have been tried to reduce that problem but despite the claims of some cat manufacturers, the prop runs in aerated water and is not as efficient. The prop has to be lowered to run in "hard" water but this increases draft.
​

​

This is where the tunnel is clearly superior. Not only does the water coming out of the tunnel rise higher than along the hull sides but it is compressed by the tunnel shape: the prop will cavitate much less than between the cat hulls.
The correct name for our type of tunnel is pocket drive. The theory is well known: the forward part of the tunnel is higher than the exit and the aerated water is compressed before it reaches the prop. At the transom, the water makes a hump and that is the level of the cavitation plate at planing speeds. For the hole shot, the prop must be deeper but once the pocket drive is "primed" the engine can rise on a jack plate allowing the boat to run in 4 or 5" of water.
(Some of the XF20 builders report running in as little as 2-1/2"!).
​

A pocket drive is not perfect: there will always be a turbulence at the mouth of the tunnel. There is simply no way around it. At planing speeds, the water going through the pocket has to cover a longer distance than the water running under the straight parts of the bottom. This will create cavitation but we can reduce its consequences with the shape of the tunnel. The tunnel is not a plain box: it is tapered in profile and all corners have large radii fillets. It is heavily reinforced to withstand the pressure variations.
The pioneer of this type of tunnel is Uffa Fox: he designed the first ones for the British Navy boats during W.W.II. Today, we know what the ideal shapes and proportions are and that is how the XF20 pocket drive was calculated.

​

I hope that helps. Ask more questions if you have them.

Tim

 

Any pics of an FX20? Sure would like to check it out.

 

Thanks Tim that is great information.

Here's what I'm trying to decide. I was planning to get a new flats boat this Spring that would allow me to get into some of the back lakes around West Bay, and suit my needs further south. As luck would have it, we have an alternate spending opportunity that will use up a lot of my new boat money. I'm trying to decide if I can do anything to my current boat that will allow it to go a little shallower.

It is a 23' Fishmaster powered my a 200 Johnson. It has a jack place (4" setback) and Nedski Foil. The rear of the hull has what I call a pocket tunnel (but maybe that's not the right term). It's a recess in the center of the transom that is about 5" high and about 9" deep (I'm estimating). On plane it creates a hump of water about 2' behind the boat similar to what was in your diagram.

With the jack plate all the way up, I can run on plane in 12", 10" with the skeg dragging. The boat will float in 9" with the motor tilted up. I've pushed it off a sandbar in 7" by myself. For as big a boat as it is, it goes pretty shallow.

Currently, with the JP all the way up, about 2/3 of the prop is below the bottom of the hull. Also, the prop is not in the highest part of the afforementioned hump of water.

What I'm wondering is if I get a new JP with an additional 2" of setback, mount the motor about 2" higher, and install trim tabs... Is it reasonable that I can get it to run 3" shallower on plane. I would also like it to plane better at slower speeds. It currently 'plows' below 15mph no matter how I trim it.

I realize I'm not going to turn it into a RFL or Shallow Sport and will lose some efficiency and maneuverability with the JP all the way up, but 3" could get me into some places that are iffy right now.

I'm really interested in hearing from some folks that really understand the hydrodynamics of this.

 

Any pics of an FX20? Sure would like to check it out.

Here ya go.
http://www.boatplans-online.com/proddetail.php?prod=XF20

 

All sounds real good, but I have certainly seen Tran Cats and Flats Cats in places where no mono hulls would dare to go. Having had both mono and cat, I'll take the cat every time for my type of fishing.

Chocolate and vanillla.

Any tunnel boat that doesn't have a full tunnel (meaning running the length of the entire hull) has a 'pocket' tunnel.

Here is a good description of the different types of hulls that run well in shallow water. It is from a study plans page for the designer of the boat I built.

Pocket drive, catamaran tunnel and some theory:​

There are different ways to design a flats boat with minimal draft. Some catamarans claim to have less draft than monohulls but that is simply not possible. ​

Let's demonstrate by comparing the geometry of two simple hulls.
The sketch below shows the waterplane area of a monohull compared to a typical catamaran hull, each simple rectangles but the comparison works just as well for more sophisticated shapes.

You can see that the catamaran hull as less foot print than the monohull: at equal draft, the cat displaces only half the water of the mono hull

and therefore, at equal boat weight will have two times more draft. Even if the tunnel is narrower, the cat hull will always need more draft but there is more: a cat structure is more complicated and the hull area is larger, therefore heavier. ​

A cat type tunnel will always have more draft because it has less waterplane area and a heavier structure.

That is for static draft but what happens when running?​

There again the monohull is superior.
The water between the hulls of the cat is turbulent. All kind of steps and other contraptions installed between the hulls have been tried to reduce that problem but despite the claims of some cat manufacturers, the prop runs in aerated water and is not as efficient. The prop has to be lowered to run in "hard" water but this increases draft.

​

This is where the tunnel is clearly superior. Not only does the water coming out of the tunnel rise higher than along the hull sides but it is compressed by the tunnel shape: the prop will cavitate much less than between the cat hulls.​

The correct name for our type of tunnel is pocket drive. The theory is well known: the forward part of the tunnel is higher than the exit and the aerated water is compressed before it reaches the prop. At the transom, the water makes a hump and that is the level of the cavitation plate at planing speeds. For the hole shot, the prop must be deeper but once the pocket drive is "primed" the engine can rise on a jack plate allowing the boat to run in 4 or 5" of water.
(Some of the XF20 builders report running in as little as 2-1/2"!).

A pocket drive is not perfect: there will always be a turbulence at the mouth of the tunnel. There is simply no way around it. At planing speeds, the water going through the pocket has to cover a longer distance than the water running under the straight parts of the bottom. This will create cavitation but we can reduce its consequences with the shape of the tunnel. The tunnel is not a plain box: it is tapered in profile and all corners have large radii fillets. It is heavily reinforced to withstand the pressure variations. ​

The pioneer of this type of tunnel is Uffa Fox: he designed the first ones for the British Navy boats during W.W.II. Today, we know what the ideal shapes and proportions are and that is how the XF20 pocket drive was calculated.

I hope that helps. Ask more questions if you have them.

Tim

 

That looks like the microdraft 20 at the site www.plugsandslugs.com this guy builds shallow running wood boats.Click on photos and captains toys.

 

All sounds real good, but I have certainly seen Tran Cats and Flats Cats in places where no mono hulls would dare to go. Having had both mono and cat, I'll take the cat every time for my type of fishing.

Chocolate and vanillla.

Well, I'm not going to get in a pi$$ing contest with you, but the physics don't lie. A cat hull can not FLOAT in less water than a similarly sized flat bottom. Even ol' Stewy has admitted that his TransCat will only Float in about 6-7 inches. These boats (XF-20) will actually float in 2-3 inches. I have not been in a RFL, but it should be pretty close to an XF-20 in static draft just from the hull shape.

I agree that the cat hulls will run pretty much where a XF or RFL will and will do it with a better ride too.

daddyjaxxs,
Yes it is very similar to a MicroDraft. The co-designer used the Henderson skiff as his "pattern" if you will. He even went and talked to Capt. Henderson during the design phase.

Gottago,
It surprises me that your Flatsmaster will float in 8-9 inches of water. I'm not doubting you as you've hopped out to measure/push, just impressed.

I think what you're asking could be tried without too much expense and I think it might help. I would not go through the trouble of replacing the jackplate if the one you have now still goes up and down well. (If you do though I might want first shot at buying it.) You can buy or have made some 2 or 3 inch spacers to get the extra setback you're talking about though. I would start there and raise the motor 1/2 to 1 inch for starters. Do you have a water pressure gauge? If not, buy one before you start all this for some baseline data.

Now with the motor further back and up a bit more see how your prop works. Talk to LouieB from this point and see what he can do for you. The higher you go the more cupping you'll need.

As far as tabs go, I can recommend a cheaper option there too. I recently put a set of SmartTabs (http://www.iboats.com/mall/?keywords=smart%20tabs&cart_id=654551110&session_id=932117656&view_id=39283) on my duck boat and was totally amazed. I've never operated or even been on a boat that had tabs so I was a bit skeptical about how they'd help. My boat now doesn't even lift the bow when taking off, just flat gets on plane. I can also keep it on plane at slower speeds like you want to do. So for only $150 +/- instead of $500+ you can get some tabs.

Now I don't know the back lakes off West Bay very well (only been there twice and briefly) so let me ask you this. If you can get you boat to run in 10 inches or less and get back there further, can you get back on plane and get out later? What if the tide is falling and you don't notice it, is there a deep hole or ditch where you're planning on going to get up in? Can you boat plane with the motor all the way up on the plate right now? Since I don't know the area, I can't see the benefit of running shallower there.

Tim

 

Tim,

Thanks for the feedback. I was looking at it this weekend and had the same idea about the spacers. I also have another hole on the motor that will get it up another 1.5" so that's an easy fix. I was also going to get Baumann to do my prop. I've looked at the smart tabs and heard good things. I'm still leaning toward the Lenco's because of the size of the boat and the extra control I can get with them.

As for the hole shot, I have corkscrewed it up in 12" over soft mud, but to answer your question, I can usually find 18" of water to get it up in. Otherwise, I just have to idle out to deeper water with the motor trimmed up.

If I decide to get another JP, I'll keep you in mind.

Thanks for your thoughts.

 

Don't forget that as you move your motor back, your center of gravity will shift aft and cause more draft and change how your boat rides. It would be best to find someone with a similar setup and see how it works. More setback is not always the answer. Also maybe a low water pickup will let you get the motor up higher.

 

Don't forget that as you move your motor back, your center of gravity will shift aft and cause more draft and change how your boat rides. It would be best to find someone with a similar setup and see how it works. More setback is not always the answer. Also maybe a low water pickup will let you get the motor up higher.

That's a good thought on the CoG. I hadn't thought about that.

I was looking at CMC's web site and they make a couple of bolt-ons that allow you to raise and/or set your motor back using your existing jack plate. I'll start with the 5" Vertical Extension and see how that does.

This may be a little trial and error, but I'll just take it one step at a time.

 

The genereal rule is for every 1" setback motor can go up 3/8". Thought I put my $.02 in.

 

I think the the Fishmaster is a stepped transom not a tunnel at all if I recall (like most bass boats )?

 

Stepped transom is probably the right term. Regardless, it seems to throw up a mound of water behind the boat on plane. I'm trying to take advantage of that as much as possible.